Allylphosphonic acid compounds have been used in a variety of applications such as, for example, flame retardants, plasticisers, lubricants, surfactants, water-treatment additives and as intermediates for various medicinal and agricultural products. Soviet Union Patent, SU 598,907, for example, discloses allylphosphonic acids which are useful as fire resistant polymers and are prepared by reacting alkali hypophosphite with allene to form the corresponding allylphosphonate alkali salt which is hydrolyzed to the allylphosphonic acid.
Typically, allylphosphonic acids are prepared by hydrolyzing corresponding esters of allylphosphonic acid in the presence of an acid catalyst. Esters of allylphosphonic acid have been prepared in a number of methods. German Patent DE 2,002,809 discloses allylphosphonic acid esters which have been prepared by a standard Arbuzov reaction of trialkylphosphite with allylbromide. This reaction provides dialkyl allylphosphonate in good yield; however, allylbromide is very expensive and presents health and ecological concerns as a volatile, highly reactive alkylating agent.
Allyl chloride has been substituted for allyl bromide in the Arbuzov reaction of a trialkyl phosphite and an allylhalide. For example, U.S. Pat. No. 4,017,564 issued to Arend et al. discloses a process for the production of allylphosphonic acid esters by reacting allyl chloride with trialkyl phosphite in the presence of an avalent and/or monovalent nickel catalyst. Yields obtained by this method are only moderate and a purification step is required to remove the transition metal catalyst. Allyl phosphonate esters have been prepared in poor yields by the reaction of allyl chloride with a sodium dialkyl phosphite. This process is described in Kosolapoff, J. Am. Chem. Soc., 73, 4040 (1951). In addition to unsatisfactory yields, prior art processes requiring allyl chloride present a hazard, since allyl chloride is very toxic and extremely volatile.
Allylphosphonate esters have also been prepared by several isomerization methods using a mixed tri-(allyl, alkyl) phosphite. In a method, as described in Lemper et al., Tet. Lett., 3053 (1964), the rearrangement has been carried out thermally at temperatures of greater than 200.degree. C. The mixed phosphite has also been rearranged under triplet-sensitized photolysis in a quartz reactor using the procedure as described in Bentrude et at., J. Am. Chem. Soc., 109 (1987). The transformation has also been accomplished using nickel(0) catalyst as described in Lu et al., J. Organomet. Chem., 304, 239 (1986). These procedures are disadvantageous since they require special reaction conditions (i.e. high temperature, photolysis) or a transition metal catalyst. In addition, these procedures require the preparation and isolation of a mixed phosphite ester which preparation requires the use of potentially explosive phosphorochlorodites.
Trialkyl phosphites have been reported to undergo ester exchange with an allylic alcohol, see for example, Lemper et al., Tet. Lett., 3053 (1964). The use of an acid catalyst in such a process has also been described in Holy, Chem. and Ind., 721 (1965). The simultaneous transesterfication/rearrangement approach has also been reported for the conversion of triphenyl phosphite into an alkyl phosphonate, see for example, Laughlin, J. Org. Chem., 27, 3644 (1962) or Honig et al., J. Org. Chem., 42, 379 (1977). Such processes however, required reaction conditions of prolonged heating at temperatures in excess of 200.degree. C.
It is one object of the present invention to provide a novel process for the rearrangement of triallyl phosphite to diallyl allylphosphonate ester. The process permits the rearrangement of triallyl phosphite under mild conditions without the use of high temperatures, photolysis, a transition metal catalyst or allyl halide.
Another object of the invention is to provide a simple and economical process of producing allylphosphonic acid from trialkyl phosphite in high yield and selectivity using inexpensive and readily-available starting reagents.
Other objects of the present invention will be evident from the ensuing description and appended claims.